1. Field of the Invention
The present invention relates to photocatalytically active (PCA) coating compositions containing a photocatalytically active oxide of a transition metal (MO) or (MO2) such as titanium dioxide (TiO2) or zirconium oxide (ZrO2) catalyst for producing clear self-cleaning coatings, such as for glass windows, which react with and decompose organic compounds or pollutants, deposited thereon from the environment, under the effects of exposure to sunlight, particularly the ultraviolet radiation contained therein. The organic pollutants are decomposed to simple inorganic compounds such as CO2, H2O and various mineral acids, which re-enter the atmosphere and/or wash away under the effects of heat, wind and/or rain, so that the coatings are self-cleaning with an efficiency which is dependent upon the degree of photocatalytic activity of the MO2 catalyst, which is directly proportional to the total surface area of the MO2 particles to which the pollutants are exposed.
2. State of the Art
It is well known that when a metal oxide, such as anatase Ti02 powder, is illuminated by ultraviolet light with a wavelength below about 390 nm, electrons in the valence band are excited to the conduction band leaving behind positive-charged holes which are reactive with absorbed water vapor hydroxide ions, resulting in the formation of positive-charged hydroxyl radicals, (OH)+. The hydroxyl radicals are strong oxidizing radicals which can react with and strip electrons from the organic pollutants to produce simpler, non-offensive products such as CO2 and H2O, or HCL if halogen pollutants are involved.
One commercially-available TiO2 powder photocatalyst is Degussa P25, a 70:30% anatase/rutile mixture with a BET surface area of 55±15 m2g−1 and crystalline sizes of 0.1 nm in 30 nm aggregates. It forms an aqueous suspension in dilute alcohol which forms a chalky catalytic coating on glass. Reference is made to the article titled “Photocatalytic Degradation Of A Gaseous Organic Pollutant” by Yu et al., published in the Journal of Chemical Education, Vol. 25, No. 6, June 1998.
It is desirable to produce a TiO2 composition which can be applied to surfaces such as window glass and dried under ambient conditions to form clear, self-cleaning photocatalytic coatings. Reference is made to articles by H. Ichinose et al. in the Journal Of The Ceramic Society Of Japan, titled “Synthesis Of Peroxo-Modified Anatase Sol From Peroxo-Titanic Acid Solution”, Vol. 104, pages 914-917 (1996), and “Photocatalytic Activities Of Coating Films Prepared From Peroxotitanic Acid Solution-Derived Anatase Sols”, Vol. 104, No. 8, pages 715-718 (1996). These articles describe a process to put small amounts (0.85% to 1.7%) of various forms or shapes (polymorphs) of titanium dioxide (Ti02) into aqueous solution by reaction with hydrogen peroxide These solutions are called titanium peroxidases-Ti0 (00H)2. The amorphous titanium dioxide is the ingredient that results in the film-forming and adhesive characteristics of the product. The mixture is composed of equal weights of the amorphous and anatase (crystalline) forms of titanium dioxide, is soluble in water in up to about 2% by weight of the composition and can be applied at ambient conditions.
U.S. Pat. No. 6,107,241 (Ogata et al.) and U.S. Pat. No. 6,429,169 (H. Ichinose) disclose an anatase titanium oxide sol having a pH of 7.5 to 9.5 and a particle size of 8-20 nm which is a yellow suspension made by adding aqueous ammonia or sodium hydroxide to a titanium salt solution, such as titanium tetrachloride, washing and separating the formed titanium hydroxide, treating the formed titanium hydroxide with aqueous hydrogen peroxide solution, and heating the formed stable amorphous titanium peroxide sol having a concentration of about 2.9%, a pH of 6.0 to 7.0 and a particle size of 8 to 20 nm and a yellow transparent color to a temperature of 100° C. or higher to form an anatase titanium oxide sol. The anatase titanium oxide sol can thereafter be heated to 250° C. or higher to convert it to anatase titanium dioxide.
The amorphous titanium peroxide sol has good bonding strength but poor wetability for substrates and is not photocatalytic and is yellowish in color. The anatase titanium formed by heating the amorphous titanium peroxide sol to elevated temperatures is photocatalytic. Therefore mixtures of the amorphous titanium peroxide sol and the anatase titanium oxide sol are made to provide a mixed sol coating composition to which may be added more photocatalyst, such as titanium dioxide in sub-10 nanometer particle size powder form, and other inert additives such as inorganic and organic binder materials, which are clear and compatible with the peroxotitanic sol so as not to alter the pH or the clarity of the solution. Even small amounts of Ti02 or other ingredients having particle sizes about about 10 nanometers will render the composition opaque and unsatisfactory for use as self-cleaning coatings on glass or other transparent substrates. The coating must be applied in the form of several layers or dips to provide adequate bonding but the end result is that the yellowish color of each layer is intensified to produce an unsatisfactory appearance on window glass. Multiple layers are necessary because the peroxide-forming film is very hydrophobic so that the coating composition does not have good wetting properties for glass and tends to bead on glass, leaving “holidays” or uncoated areas and requiring multiple overlayers.
A process of producing both an amorphous titanium peroxide solution in water and also anatase particles in the range of 6 to 10 nanometers is described in U.S. Pat. Nos. 6,107,241 and 6,429,169. The amorphous titanyl peroxide forms an insoluble film when the peroxide breaks down or reacts with water. This serves as a carrier for the anatase particles.
The application of the film independently, or with the particles embedded, when applied to glass, plastic or metal has the following problems.
1. The film former is very hydrophobic and does not wet out to form a continuous film. A heavy amount or thick layer of the composition is required to form a continuous film or covering . The surface tension or the peroxide-containing film is to some degree overcome by the added thickness and weight of the film. The time and labor for such application makes the use of the product impractical.
2. The film is formed with difficulty, and is yellowish in color due to the presence of the titanyl peroxide remaining and unreacted. This is aggravated if the weight and thickness of film is increased to overcome the surface tension of the titanyl peroxide solution to form a continuous coating on the substrate.
3. Transparency and clarity of the coating(s) when applied over glass is impaired due to the thickness required to overcome the non wettability of the substrate. The refractive index of the film so produced and the excessive thickness causes moire patterns and a seemingly rainbow effect when viewed through clear glass.
The photo-chemically active component is the anatase polymorph. The peroxytitanic acid polymorph has no photochemical activity. The photo-chemically active polymorph is derived by heating the amorphous titanyl peroxide sol at 100 degrees centigrade temperature for six hours.
The peroxytitanic acid polymorph has a yellow coloration that remains in the product even when it is mixed with the titanium peroxidase. This yellow coloration is objectionable on clear window glass. However, the solubility of the TiO2 is related to the addition of the peroxide; without the peroxide, the TiO2 does not go into solution. Consequently, it is highly desirable, and necessary for many uses, to remove entirely, or to reduce as much as possible, the yellow coloration, to provide clear, self-cleaning window coatings.